This invention relates to a toner concentration-detecting apparatus for a developer comprising a magnetic carrier and a toner in a developing apparatus.
Methods for detecting the concentration of the toner of a two-component developer comprising a magnetic carrier and a toner by utilizing variations in inductance are suggested in the Japanese Patent Publication No. 8280/71 and U.S. Pat. No. 3,802,381. According to these methods, variations in magnetic permeability of the developer in accordance with the mixing ratio of the magnetic carrier and the non-magnetic toner are detected in the form of variations in coil inductance.
A two-component developer comprising a mixture of magnetic carrier such as iron powder and black toner powder is usually used with a magnetic brush for development. The magnetic field formed by a magnet making up the magnetic brush is apt to act on toner concentration-detecting coils, resulting in an error in coil induction detection.
Although the toner concentration can be measurable in theory by embedding the detecting coils in the developer for detection of induction variations, the toner concentration-detecting means has, in actual fact, been placed under the influence of the magnetic field formed by the roll magnet making up the magnetic brush. Generally, the magnetic brush is comprised of a fixed magnet on the outer periphery of which a sleeve is rotated to supply the developer onto a photosensitive surface. For this purpose, the sleeve is generally made of a non-magnetic metal such as brass, stainless steel or aluminum. With the rotation of the sleeve, eddy currents occur, thereby often causing variations in magnetic field in the developer. Therefore, the mere embedding of coils in the developer case or container is not enough for correct measurement of inductance.
Further, the magnetic field formed by the magnet making up the magnetic brush magnetically saturates the magnetic carrier. The difference in saturation degree due to the temperature characteristics of the magnet or changes thereof with time and the resulting variations in inductance are detected as an error.
Preferably, the detecting coils or the contact area of the coils with the developer should be larger to enable toner concentration detection over a larger portion of the developer. Nevertheless, each coil should be of an appropriate size in order to assure the compactness of the detecting apparatus and the developer container.
Furthermore, the fact that the detecting coils are located within the developer container disturbs homogeneous circulation flow of the developer, inconveniently resulting in a lack of uniformity of the magnetic brush.
Some of the conventional apparatus for detecting the toner concentration of the developer such as disclosed in U.S. Pat. No. 3,572,551, in order to obviate the above-mentioned disadvantages, are so constructed that the detecting coils are not placed in the developer container but part of the developer is taken out of the developer circulation path in the developing apparatus to measure the toner concentration at a point separate from the container not affected by the above-mentioned problems. This type of apparatus, however, is bulky and complicated, and has the shortcoming of low reliability.